1. Field of the Invention
The present invention relates to communication networks, and more particularly to a communication network including a single master station and a plurality of slave stations.
2. Description of the Background Art
In these years, Local Area Networks (hereinafter referred to as LANs) are in use, which are data communication systems in which a large number of independent devices can directly communicate with one another within an adequate distance through a physical link having a prescribed data rate. Media access controls in the LANs include the token passing. FIG. 31 is a diagram illustrating the structure of a token ring LAN adopting the token passing and the data communication in that LAN. In FIG. 31, stations 3101, 3102 and 3103 are connected to a ring-shaped bus 3104 to communicate with one another. These stations 3101-3103 are previously provided with ID information in an un-overlapping manner for specifying each station. The data communication in this LAN will now be described referring to FIG. 31.
In the LAN, as shown in FIG. 31(a), a token 3105 circulates along the bus 3104. The format of this token 3105 includes, as shown in FIG. 32(a), a starting delimiter 3201, an access control field 3202 and an ending delimiter 3203.
Each station once acquires the token 3105 coming from the bus 3104. When having no data to transmit at this time, however, it sends out the token 3105 onto the bus 3104 toward a station located on the downstream. This operation is continuously repeated, so that the token 3105 is constantly circulating on the bus 3104 in a certain direction when no station has data to transmit
As shown in FIG. 31(b), the station 3102 currently holds transmission data 3106. As shown in FIG. 32(b), the frame format of the transmission data 3106 includes destination address 3204, source address 3205, information 3206, frame status 3207 and other necessary control information (starting delimiter, access control field, frame control, frame check sequence and ending delimiter). The station 3102 acquires and holds the coming token 3105 and then, as shown in FIG. 31(c), sends out the transmission data 3106 onto the bus 3104. It is assumed that this transmission data 3106 contains ID information for the station 3101 as the destination address 3204.
The station 3103 located downstream of the station 3102 acquires the coming transmission data 3106. The station 3103 then determines whether the ID information of the destination address 3204 included in the transmission data 3106 agree with the ID information assigned to itself. In the station 3103, the two pieces of ID information disagree. In this case, the station 3103 sends out the transmission data 3106 onto the bus 3104.
Next, as shown in FIG. 31(d), the station 3101 acquires the transmission data 3106 and operates similarly to the station 3103. In the station 3101, the ID information stored in the destination address 3204 agrees with the ID information assigned to it. In this case, the station 3101 copies the information 3206 included in the transmission data 3106 and indicates reception of the information by using a frame copy bit included in the frame status 3207. Then the station 3101 sends out the transmission data 3106 onto the bus 3104.
The transmission data 3106 circulates around the bus 3104 and returns to the station 3102, as shown in FIG. 31(e). The station 3102, knowing that it has sent out the transmission data 3106, detects the head of the frame to know that the transmission data 3106 sent by itself has returned. The station 3102 then checks the frame copy bit included in the frame status 3207 to know that the station 3101 has received the data. Thus, knowing that the data communication to the station 3101 has been normally achieved, the station 3102 sends out the currently held token 3105 onto the bus 3104, as shown in FIG. 31(f). A series of operation for data communication are thus ended.
Recently, with the digitization of picture data and sound data, not only are computers connected to the LAN but various AV (Audio Video) equipment handling picture data and sound data are also connected to the LAN. This has brought about mixed existence of two kinds of data with different natures in the LAN: Asynchronous data and Isochronous data. That is to say, the AV equipment perform communication of computer data such as AV equipment control commands, or the Asynchronous data for controlling the data communication, and they further perform communication of digital AV data image-compressed by MPEG (Moving Picture Experts Group) and the like, or the Isochronous data. The digital AV data (Isochronous data), having a highly real-time nature, that is, having continuity in time, requires that the communication should be done certainly within a certain time period.
In conventional LANs, however, the token is sent around even to stations having no transmission data and a station which has acquired the token keeps it until it confirms that the data transmitted by itself has been received by a destination station. If a certain station keeps the token arrested with a plurality of stations transferring Isochronous data, other stations can not transfer the Isochronous data. Then the real-time nature of Isochronous data held in other stations are not satisfied, causing problems in the data communication.
Although not the above-mentioned token ring LAN, a single master station for controlling transmission of tokens and a plurality of slave stations only for data communication are connected to a Master Slave Network, where each station analyzes a coming token and then sends out the token onto the bus. FIG. 33 is a diagram illustrating the analysis of the token 336 in each station.
In FIG. 33, in the Master Slave Network, a master station 331a and slave stations 331b, 331c and 331d are connected to the ring-shaped bus 335 so that they can communicate with each other. The master station 331a includes a token transfer and analysis executing device 332a and a storage device 333a. The slave stations 331b-331d include analysis executing devices 332b-332d and storage devices 333b-333d.
The token transfer and analysis executing device 332a sends out the token 336 onto the bus 335 through the storage device 333a. The token 336 is once stored in the storage device 333b of the slave station 331b and then analyzed by the analysis executing device 332b. When this analysis is finished, the token 336 is transferred from the slave station 331b to the bus 335 again and inputted to the slave station 331c. The analysis executing device 332c too applies such analyzing process as performed in the analysis executing device 332b to the token 336 stored in the internal storage device 333c. Such analyzing process is similarly performed in the slave station 331c as well and the token 336 is finally stored in the storage device 333a in the master station 331a. The token transfer and analysis executing device 332a applies the analyzing process described above to the token 336 stored in the storage device 333a and then sends out the analyzed token 336 onto the bus 335.
In the conventional Master Slave Network, the analysis executing devices 332b-332d and the token transfer and analysis executing device 332a apply analyzing process to the token 336 stored in the storage devices 333b-333d and 333a. That is to say, it takes the time required for transfer on the bus 335 plus the time required for analyzing processes in the slave stations 331b-331d. Accordingly, it takes a long time before the token 336 comes back to the master station 331 a. Similarly, data sent out by a slave station which has acquired the token 336 also takes a long time to circulate around the bus 335. Since this problem becomes more serious as a larger number of stations are connected, high utilization efficiency and high response can not be expected with a large number of stations connected in the Master Slave Network. To solve the problem, it is suggested to improve the utilization efficiency and response speed by causing the storage devices and analysis executing devices to operate at higher speeds or by reducing the number of stations connected to the network. However, remarkable effects are not expected.